The present invention relates to a method of producing fine metal balls such as very small solder balls used to electrically connect integrated circuits (ICS).
Conventional methods of manufacturing fine metal balls having a low melting point include 1) an atomization method and 2) a method involving dipping metal pieces into a heated liquid. In the Japanese Unexamined Patent Publication No. 50-102558 there is proposed a method for manufacturing fine metal balls which comprises the steps of printing paste, which contains low-melting-point metal powder, onto a metal plate through a porous plate to thereby partition the paste into a predetermined amount, and then heating them.
Although the atomization method is suitable for producing a large number of particles, it is not suitable for shaping particles into balls or for making particles with the same intended size. On the other hand, the method involving dipping metal pieces into a heated liquid can produce particles with substantially spherical shapes and increase the accuracy in the size of balls by using metal pieces of the same size prepared beforehand. However, this method is unsuitable for mass production due to poor work efficiency and poor productivity factors as it is necessary to maintain a large space between metal particles so that molten metal particles do not stick or do not join together when contacting each other in a liquid and as a period of time is required to cool the liquid down to a temperature less than the melting point of the metal so that the molten metal particles can be solidified while retaining the shape of a ball.
Although the method disclosed in the Japanese Unexamined Patent Publication No. 50-102558 is suitable for mass production, it is difficult to provide balls having a precise size due to the distortion of the balls into a flattened shape in a case where the diameter of the balls is large. It is also difficult to accurately partition metal having a low melting point due to the use of a printing process, and to obtain the constant dimension accuracy of fine balls due to uneven blending of metal particles. Because of these problems, the diameter of balls is disadvantageously limited to 0.1 to 1.0 mm.
The prior art does not satisfy all of the following requirements: controllability for providing metal balls with a uniform diameter; providing a perfect ball shape; mass-producibility; and permitting a wide range of ball sizes.